Probably the best known example of the use of the behaviour of a penetrating body in a material to determine its properties is the measurement of material hardness. Here there exist a considerable number of methods, for example, Shore Hardness measurement, Vickers Hardness measurement, Rockwell Hardness measurement, Brinell Hardness measurement, Herbert's pendulum etc. In view of the fact that the surface layer is fully or partially destroyed, however, these methods cannot be used in a great many areas of application. For example, if the hardness of paint is to be measured, it may be the case that the paint layer is not completely penetrated. Despite this the paint is damaged and it is possible that a rust spot at the test point could form the nucleus of local corrosion, whereas the rest of the paint layer is still in order. For this reason, the finished object is not measured, rather a test specimen. The test specimen, however, is not the finished object.
There are also non-destructive methods for measuring the properties of solid materials which can be derived from their behaviour under penetration.
It is, for example, known that by placing an ultrasonic probe on a material the frequency mismatch can be measured. This method is very sensitive to the degree of coupling with the test object. In addition, it is not the hardness alone which is measured, because the measured value depends substantially on the modulus of elasticity of the material, the thickness of the coating and the shape of the object, in particular on the thickness. The method can only be used for comparative assessments and not for absolute measurements.
A further non-destructive method giving absolute measurements was described in the "Industrie Anzeiger" of Dec. 2, 1981 under the (transalted) title "Method for Testing the Case Hardness Distribution of Forgings". The device described is very expensive. It is also so large that the object must be brought to the device; the reverse is not possible. A force is applied inductively and the depth of penetration measured by optical displacement measurement. Due to the sensor system employed for measurement, it is not possible to safely determine the depth of penetration in the micrometer range. The method is designed for measurement of case hardness with typical penetration depths of 500 .mu.m. Similar to the case of a micrometer screw gauge, the system requires a very rigid yoke. The depth of penetration at a given force is converted by a difficult-to-use nomogram, since the measured value is a complicated function of the force applied.